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SMS  for  Remote  Data  Collec5on  

Mar5n  Saint  mar$n.saint@colorado.edu  

itp.colorado.edu  (with  Suzana  Brown  and  Professor  Timothy  X  Brown)  

 University  of  Colorado  Boulder  

Interdisciplinary  Telecommunica$ons  Program  (ITP)  College  of  Engineering  and  Applied  Science  

Purpose  and  Scope  

•  This  presenta$on  will  focus  on  tes$ng  characteris$cs  of  the  Short  Message  Service  (SMS)  protocol  from  mobile  devices.  

•  While  SMS  has  been  around  since  the  mid  1980s,  there  is  a  lack  of  empirical  performance  data.  

•  Tes$ng  was  undertaken  around  a  U.S.  Federal  Communica$ons  Commission  (FCC)  ini$a$ve  to  permit  text  messaging  for  contac$ng  emergency  services,  but  SMS  has  many  other  applica$ons.  

•  Details  here  are  specific  to  GSM  based  systems,  but  similar  concerns  affect  those  based  on  CDMA.  

•  Only  a  few  example  tests  will  be  presented  today.  

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Wireless  Access  Op5ons  

•  Needed  to  connect  remote  device(s)  to  the  core  network  (unless  a  wired  solu$on  is  chosen).  

•  Dependent  upon  availability,  required  performance  for  applica$on,  cost.  

•  Some  op$ons:  • 2G,  3G  Cellular  (data  x.25,  x.32,  GPRS,  SMS)  • WiMAX,  LTE  • Microwave  • Satellite  • What  can  you  think  of?  

•  Long-­‐link  802.11  Wi-­‐Fi  

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Advantages  and  Disadvantages  of  SMS  

Advantages  

•  Widely  available  

•  Widely  used,  well  understood  

•  Inexpensive  (rela$vely)  

•  Simple  

•  Reliable  •  But  not  always  $mely,  especially  when  sending  to  a  mobile  receiver.  

•  Has  the  advantages  (and  disadvantages)  of  typical  wireless  networks  

Disadvantages  

•  Store  and  forward  (not  always  $mely)  

•  160  character  limit  •  But  mul$ple  messages  possible.  

•  Mul$ple  messages  may  arrive  out  of  order  

•  SMS  itself  provides  no  loca$on  informa$on  

•  No  confirma$on  of  delivery  

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GSM  Mobile  System  Architecture  

J.  Scourias,  “Overview  of  the  Global  System  for  Mobile  Communica$ons,”  14-­‐Oct-­‐1997.  [Online].  Available:  hjp://ccnga.uwaterloo.ca/~jscouria/GSM/gsmreport.html.  [Accessed:  22-­‐Feb-­‐2012].  

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Frequency  Bands  and  Channel  Structure  

•  Frequencies  are  licensed  and  depend  upon  the  country.  • Common  frequencies  around  850,  900,  1800,  1900  MHz  

•  Channels  • Control  channels  are  used  for  call  setup,  power  levels,  etc.  • Traffic  channels  carry  the  actual  voice  and  data  • Except  for  SMS,  which  are  sent  over  GSM  Control  Channels  

•  The  160  character  message  limit  is  based  in  the:  • Observed  length  of  wrijen  messages  when  the  standard  was  created  

• The  limita$ons  of  the  control  channel  • The  character  set  used  for  encoding  

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Frequency  Bands  and  Channel  Structure  (con5nued)  

•  GSM  uses  FDMA  to  divide  the  allocated  spectrum  into  200  kHz  physical  channels.  

•  Each  of  these  is  further  divided  into  8  $meslots  using  TDMA  • Each  of  these  divisions  is  further  divided  into  logical  channels  with  different  purposes  (which  are  not  discussed  today)  

• Logical  channels  include  the  control  and  traffic  channels  

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Frequency  Bands  and  Channel  Structure  (con5nued)  

•  One  26  frame  mul$-­‐frame  per  TDMA  $meslot  

J.  Scourias,  “Overview  of  the  Global  System  for  Mobile  Communica$ons,”  14-­‐Oct-­‐1997.  [Online].  Available:  hjp://ccnga.uwaterloo.ca/~jscouria/GSM/gsmreport.html.  [Accessed:  22-­‐Feb-­‐2012].  

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Large-­‐scale  Path  Loss,  Small-­‐scale  Fading,  Mul5-­‐path  

•  Our  project  was  designed  to  measure  characteris$cs  of  system  performance  “at  the  limits.”  

•  GSM  standard  specifies  minimum  RX  level  of  -­‐105  dBm  

A.  F.  Molisch,  Wireless  communica.ons.  Weinheim:  Wiley,  2011.  

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Lab  Equipment  

•  Telit  EVK2  GSM  Developer’s  Evalua$on  Kit  with  manuals  (mobile  phone  board)  

•  Power  supply  •  SIM  card  with  a  valid  network  

subscrip$on  (Corr  Wireless  on  AT&T/Cingular  network)  

•  Larsen  Special  remote  mobile  antenna  

•  20  dB  fixed  bullet  ajenuator  pad  •  S.M.  Electronics  SA3550S  manual  

step  ajenuator,  0-­‐3000  MHz,  50dB,  1  dB  step  

•  Mini-­‐Circuits  15542  Splijer  •  Type  N  connectors/adaptors  

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Lab  Equipment  (con5nued)  

•  SMA  connectors/adaptors  •  LMR-­‐240  coaxial  cables  •  Anritsu  Spectrum  Master  

MS2721B  spectrum  analyzer  •  Agilent  ESG  signal  generator  •  Personal  computer  running  

Windows  7  with  PuTTY  terminal  emulator  

•  USB  to  DB9  (RS232)  serial  cable  (PC  to  Telit  EVK2  Interface)  

•  Mobile  telephone  on  Sprint  CDMA  network  

•  Landline  telephone  connected  to  PSTN  

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Schema5c  Equipment  Setup,  and  The  Tests  

Telit  Board(mobile  phone)

S

1          Splitter        2Attenuator  

Pad(20dB)

Variable  Step  

Attenuator

Spectrum  Analyzer

•  Delay  of  various  size  messages    •  Reliability  and  delay  of  two  different  message  sizes  at  various  

signal  strengths    

•  “Choreographed  mobility,”  fading  implica$ons  for  delay  and  reliability  

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Example  Spectrum  Analyzer  Output      

Measurement  of  a  superframe   Time  to  send  an  SMS  

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Delay  of  various  size  messages  

 

 Message size

Average delay time in seconds

Standard deviation in seconds

1 char 3.28 0.29

60 char 4.06 0.38

160 char 5.59 0.38

•  Objec$ve:    establish  a  baseline  for  the  $me  delay  of  SMS  and  evaluate  messages  of  different  sizes.  

•  Delay  is  defined  as  the  $me  to  transport  the  SMS  from  the  MS  to  the  BTS,  the  end-­‐to-­‐end  message  delay  is  not  inves$gated.  

•  The  overhead  of  an  SMS  message  is  large  and  message  size  does  not  considerably  increase  delay.  

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Reliability  and  Delay  at  Various  Signal  Strengths  

 

 

•  Objec$ve:    evaluate  the  effects  of  very  low  signal  strength,  such  as  when  a  MS  is  heavily  obstructed  or  at  the  edge  of  coverage.  

•  Introduce  variable  ajenua$on  to  reduce  the  strength  of  the  signal  sent  and  received  by  the  MS.  

Signal level Average delay for 60- char message

Standard deviation for 60-char message (s)

-97 dBm (strong) 4.06 0.38

-109 dBm (weak) 4.49 0.76

Signal level Average delay for 160-char message

Standard deviation for 160-char message (s)

-97 dBm (strong) 5.59 0.38

-109 dBm (weak) 6.19 0.93

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Cumula5ve  Distribu5on  Func5on  

•  Delay  is  not  normally  distributed,  so  we  used  the  data  to  derive  an  empirical  cumula$ve  distribu$on  func$on  to  make  a  predic$on,  with  a  given  probability,  on  the  $me  needed  to  transmit  a  message  of  a  specific  size.    

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Fading  Implica5ons  for  Delay  and  Reliability    

•  “Choreographed  mobility”  [1].    The  MS  antenna  is  moved  in  a  choreographed  manner  to  simulate  mobility  while  concurrently  sending  a  series  of  60-­‐character  messages.  

•  Used  low  signal  strength  of  -­‐109  bBm  and  traced  a  path  of  approximately  eight  meters  in  random  fashion.  

Rensfelt,  O.,  Hermans,  F.,  Larzon,  L.,  Gunningberg,  P.,  2010.  “Sensei-­‐uu:  a  relocatable  sensor  network  testbed”  Proceedings  of  the  fi6h  ACM  interna.onal  workshop  on  wireless  network  testbeds,  experimental  evalua.on  and  characteriza.on,  ACM,  New  York,  NY,  USA,  63-­‐70.  DOI=10.1145/1860079.1860091    

SMS 60 char Average in s Standard deviation in s stationary 4.09 0.38 mobile 5.24 1.66

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Fading  Implica5ons  for  Delay  and  Reliability  (cont.)    

•  As  in  the  previous  situa$ons,  the  mean  of  the  delay  $me  of  a  mobile  case  increases  slightly  as  compared  to  sta$onary,  but  the  standard  devia$on  quadruples.  

•  The  large  standard  devia$on  implies  that  in  realis$c  situa$ons  (when  the  sending  device  is  moving)  the  $me  delay  varies  considerably.  

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Conclusions    

•  No  messages  were  lost  or  significantly  delayed  during  the  course  of  tes$ng.  • As  long  as  the  mobile  sender  could  connect  to  the  base  sta$on  messages  were  delivered  reliably  

•  Low  signal  strength  and  fading  significantly  affect  the  standard  devia$on  of  message  sending  $mes.  

•  With  a  knowledge  of  protocols  and  some  lab  equipment,  it  is  possible  to  do  original  engineering  evalua$ons  for  your  unique  applica$ons.  

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